Millions of U.S. workers have to work night shifts, resulting in serious consequences such as sleep deprivation, fatigue, reduced alertness, impaired performance, gastrointestinal disorders, and reduced job and public safety. These problems occur because the circadian clocks of the workers do not usually phase shift (reset) to adjust to night work and day sleep schedules. Thus, there is a misalignment between the physiological circadian rhythms and the work and sleep schedule. Night workers are forced to work and sleep when their bodies are not prepared for either. We have shown, in simulated night shift studies, that appropriately timed bright light during the night shift combined with specific dark periods for daytime sleep can produce complete re-alignment of circadian rhythms with the night work, day sleep schedule. However, with complete adjustment night workers would not be adapted to the night time sleep schedule that they subsequently follow on their days off. This will limit the practical application of these types of schedules. [unreadable] [unreadable] We plan to test a schedule for permanent night work that would produce sufficient re-alignment both during night shifts and during days off. Subjects will "work" in the lab and sleep at home. Treatment groups will be exposed to bright intermittent light during the night shifts and will adhere to prescribed dark/sleep episodes (for daytime sleep after night work and sleep on days off). This treatment should phase delay the circadian clock (set it later) to a compromise position, in between complete adaptation to night work and complete adaptation to days off. Control groups will remain in ordinary room light during the night shifts and will be free to sleep whenever they choose while off work. A series of experiments are proposed in which circadian phase will be assessed on different days of the schedule by measuring the circadian rhythm of melatonin. We will determine whether the compromise phase position can be achieved and maintained in the treatment groups despite alternations between series of night shifts and days off. We will determine whether more subjects in the treatment groups than in the control groups obtain a favorable compromise phase position. We will determine whether the treatment groups perform better and feel more alert on the night shift than the control groups, and whether they obtain more sleep. This work will have important implications for the health, safety, and well being of the night shift worker. [unreadable] [unreadable]